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The photoconductivity of carbon nanotubes (CNTs) has been studied theoretically in a nanotube p–n junction, a single SWNT transistor, and thin SWNT films. While individual nanotubes generate discrete fine peaks in optical absorption and emission, macroscopic structures consisting of many CNTs gathered together also demonstrate interesting optical behavior. For example, a millimeter-long bundle of aligned multi-walled nanotubes (MWNTs) emits polarized incandescent light by electrical current heating, and recently researchers in China have made multi-walled nanotubes (SWNT) bundles giving higher brightness emission at lower voltage compared with conventional tungsten filaments. Recent achievements in fabricating self-assembled centimeter-long bundles of CNTs have greatly facilitated study on the macroscopic behavior of these bundle structures. Preliminary results such as an optical polarizer and a light bulb based on CNT macrobundles have been reported.

Researchers at Tsinghua University in Beijing/PR China now have constructed a simple setup with macrobundles, up to 50 µm wide, consisting of thousands of aligned CNTs suspended between two electrodes, which can sense the incident visible and infrared light through the difference of current flowthrough the bundle.

Dr. Jinquan Wei from the Department of Mechanical Engineering, Key Lab for AMMPT of Education Ministry, at Tsinghua University, first author of a recent paper describing theses findings, explained the research to Nanowerk: "Several types of nanotubes (SWNTs, DWNTs, and MWNTs) were tested and each of them shows distinct response upon light illumination in terms of the magnitude and direction of current difference. We calculated important parameters for photosensors, such as response time and energy efficiency. Such results demonstrate that nanotube macrostructures are promising candidates for constructing infrared/visible sensors."

The paper, titled "Carbon Nanotube Macrobundles for Light Sensing", appeared in the July 28, 2006 online edition of Small.

Read the full article on the Nanowerk website.

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